Modern communication systems often have a capacity formed from the aggregation of multiple transmission bands. For example, a very high speed digital subscriber line (VDSL) comprises an upstream link (i.e., from subscriber to network) and a downstream link (i.e., from network to subscriber). Each of the links in a VDSL communication system are commonly implemented with two CAP/QAM bands as shown in FIG. 1. Hence VDSL downstream or VDSL upstream links often have two transmission bands 10, 30 and 20 and 40 respectively. However, due to the dispersive nature of twisted copper pair, severe channel attenuation as well as intersymbol interference (ISI) are unavoidable in VDSL applications. To mitigate the effects of ISI, some sophisticated form of equalization is often necessary. Furthermore, the VDSL environment consists of a wide variety of loop configurations; as a result, the optimal transmission bandwidth of one particular line may be grossly mismatched to that of another line. Therefore, typical VDSL networks optimize the transmission bandwidth on a line-by-line basis.
In a conventional single-carrier digital transmission system, such as a Quadrature Amplitude Modulation (QAM) system, implemented with an equalizer or a precoder, the transmission bandwidth is determined by the symbol rate and the carrier frequency of the system. Alternatively, some DSL systems, such as for example, ADSL often use a technology that is referred to alternately as Multi-Carrier Modulation (MCM), multi-tone, Discrete Multi-Tone (DMT), and Orthogonal Frequency Domain Multiplexing (OFDM). Hereinafter these techniques will be collectively referred to by the single name Multi-Carrier Modulation (MCM).
Referring to FIG. 2, MCM systems typically divide the individual transmission bands into a relatively large number of narrowband subchannels, with each subchannel carrying a separate QAM signal with fixed symbol rate and center frequency. In a typical MCM system each of the four bands for VDSL are filled with a large number of narrowband subchannel signals, where each subchannel signal has a fixed symbol rate and center frequency and therefore a fixed bandwidth. FIG. 2 depicts the individual narrowband subchannels as not overlapping, but in reality some overlap may occur. The resulting contamination from one subchannel into adjacent subchannels may be reduced by a combination of equalization and orthogonalization techniques.
The number of subchannels used by MCM techniques is usually much larger than can be clearly depicted in a graphical illustration such as FIG. 2. Standard proposals for MCM-based VDSL systems typically feature subchannels that are approximately 4.3125 kHz wide. This means that the number of subchannels used to implement a MCM VDSL system with a total bandwidth of 12 MHz is approximately 2782. In an MCM system, the total bandwidth used in each band can be adjusted by varying the number of subchannels that are active. For example, referring to FIG. 3 the first downstream band 10, that ranges in frequency from 0.138 MHz-3.75 MHz, has all of its corresponding subchannels activated except those in the range of 3.5 MHz-3.75 MHz]. This may sometimes be done in VDSL to avoid interference with an amateur radio band which operates in the [3.5 MHz, 4.0 MHz] region of off-air spectrum. Similarly, the subchannels occupying the 3.75 MHz-4.0 MHz region of the first upstream band 20 may also be disabled. In addition, for this example the subchannels near the transition frequency (5.2 MHz) of the first upstream band 20 may be disabled to simplify the circuitry that duplexes upstream and downstream bands onto a single copper twisted pair. Disabling subchannels near the band transition frequency allows the use of analog filters that protect the receiver Analog-to-Digital Converter (ADC) from echo and distortion caused by the local downstream transmitter.
For this example subchannels in the second downstream band 30 and the second upstream band 40 near the band transition frequencies of 5.2 MHz and 8.5 MHz may also disabled, again to allow for analog filters which can assist in the duplexing of upstream and downstream signals onto a single twisted pair. In addition, in this example, the second upstream band 40 stops transmission at 10 MHz by disabling subchannels located above 10 MHz in frequency. Typically, high frequency subchannels are disable when the twisted pair is of sufficient length that no transmission capacity is available at frequencies higher than 10 MHz. Disabling subchannels above 10 MHz in this case saves transmitter power without reducing the overall throughput available.
As shown by this example, MCM systems can modify the transmission spectrum by enabling certain subchannels and disabling the rest. In addition, some MCM systems can vary the signal constellations used on each individual subchannel. In order to achieve good transmission performance on twisted pair channels, MCM DSL transceivers typically implement algorithms that determine which subchannels are enabled, and which constellation to use on each of these active subchannels. Numerous approaches exist for accomplishing these two steps. Each of the conventional approaches assume the use of MCM modulation with its inherent fixed-bandwidth subchannels, and that overall bandwidth adjustment is therefore performed simply by deciding which subchannels to enable and which to disable. This is in fundamental contrast with single carrier modulation (SCM) systems. By definition, SCM systems contain only a single QAM channel per band. For example, a SCM implementation of the example VDSL spectral plan of FIG. 1 comprises four QAM signals, one for each of the four bands shown. By contrast, as already pointed out a MCM implementation of this spectral plan typically will require the use of approximately 2782 QAM subchannels, with hundreds of QAM subchannels dedicated to each of the four bands of FIG. 2. Because there is only a single QAM channel per band in a SCM system, it is not possible to vary the overall frequency range used within each band by enabling and disabling certain subchannels. As a result, MCM techniques for bandwidth optimization cannot be applied to a SCM system. Instead, what is needed in SCM systems is a facility for varying the symbol rate and center frequency of the single QAM signal per band. With such a facility in place a SCM system can generate a desired overall signal spectra, e.g. one matching that of the MCM system in FIG. 3, but through means of only a single QAM channel per band, as shown FIG. 4. In addition, for optimal performance across a wide variety of twisted pair channels and across differing noise environments, an algorithm is needed to select specific values for these parameters.